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Highlights: [•] The addition of paper waste increases the thermal performance of bricks. [•] Paper waste additions into brick increase the porosity. [•] Predictive models for simulation and optimization of heat transfer process have been developed. [•] Material conductivity, surface emissivities and mean brick temperature have been considered as input variables. [•] The design of hollows in brick models helps further improve the thermal performance. [Copyright &y& Elsevier]

Abstract: One of the most important factors in the thermal behavior of exterior walls is the thermal conductivity of the clay in their bricks. The higher the percentage of lightening additives incorporated, the better the thermal behavior of the bricks. But as their thermal characteristics are improved the mechanical properties of bricks deteriorate. The goal is to study the influence of paper pulp as a lightening additive on the thermal and mechanical properties of the fired clay, and to improve the thermal properties as far as possible without the mechanical properties falling below the required levels. This involved the preparation of test specimens that were then subjected to the corresponding assays of thermal conductivity and mechanical strength. It is found that around 15% of paper pulp can be added without the brick''s mechanical properties ceasing to comply with current legislation. This amount of paper pulp has improved conductivity properties by 39.69% compared to the clay without additives, recording a minimum conductivity value at 10°C of 0.45W/m-K. This decrease in the conductivity of the clay results in a 16% improvement in equivalent thermal transmittance for brick walls made in the same way. [Copyright &y& Elsevier]

• Switchable insulation is an effective means of regulating heat through envelopes. • Current research on switchable insulation is fragmented across multiple domains. • We review existing technologies systematically and propose a classification system. • We assess their performance and improvement opportunities for building applications. Switchable thermal insulation, in the form of an opaque panel that alternates between thermally conductive and insulated states, can be an effective means of regulating the thermal environment by selectively transferring heat between the indoor and outdoor environments. Pioneering work has been undertaken by researchers to develop switchable insulation technologies intended for applications in the built environment, automotive, and aerospace, where conventional space heating and cooling technologies are either too bulky or too energy consuming to meet design requirements. Switchable insulation technologies are in their infancy and the emerging research on this topic is unstructured and fragmented across disparate application sectors and very few of the adaptive insulation concepts and technologies are actively being pursued by the buildings research community. The aim of this paper is therefore to advance the understanding of switchable insulation for possible applications in the building envelope, by reviewing and classifying the existing technologies systematically, with a particular focus on their working principles, theoretical performance and improvement opportunities. The paper provides a qualitative and quantitative assessment of the most promising switchable insulation technologies for building envelopes and identifies the opportunities for further research in each of the technologies. [ABSTRACT FROM AUTHOR]

This paper explores the concept of characterizing the as-built Heat Loss Coefficient (HLC) of buildings based on-board monitoring (OBM), via energy consumption and temperature sensors, and time series analysis. It is examined (1) how the coefficients of different Auto-Regressive with eXogenous inputs (ARX) models can be interpreted and (2) whether these conclusions are sensitive to the building envelope assembly or the applied indoor temperature profile. The paper presents a theoretical case study whereby detailed building energy simulations are used to accurately map the impact of physical phenomena on the characterization process. The simulation models and boundary conditions are composed to focus on the link between the estimated ARX-coefficients and the physical driving forces for transmission heat loss to the ground and the exterior environment. The results show how the various ARX model coefficients are linked to specific components of the HLC (e.g. heat transfer through the walls and roof or through the slab-on-ground floor) and to what extent they are affected by the selection of input variables. By monitoring the ground temperature, the transmission heat losses can rather accurately be assigned to either the slab-on-ground or the walls and roof. Without this measurement data, the uncertainty on the estimates increases (ranges of the 95% confidence interval of up to 35% of the mean estimate). Modeling the ground heat losses by a constant intercept term leads to underestimations of the reference HLC of up to 59%, whereas adding heat flux sensors to monitor the transmission heat losses to the ground to the measurement set-up allows to assess the transmission heat transfer coefficient to the exterior environment HLC e within 2%. [ABSTRACT FROM AUTHOR]

Solar-based radiant heating systems represent a sustainable, and relatively low-cost, technology to raise the temperature of the interior thermal mass of our buildings. Through the use of direct absorption solar thermal collectors, the same working fluid which absorbs the solar energy can be used to transfer the energy for storage in the thermal mass of the structure using a network of pipes embedded in concrete floors. This study investigates a promising working fluid which can be used in such systems – one which is based on multi-walled carbon-nanotubes suspended in normal base fluids. A major stumbling block affecting the wide spread use of carbon-nanotube nanofluids is their low dispersion stability at elevated temperatures, which significantly reduces the absorption capabilities of the nanofluids and could lead to clogging of the pumps used to circulate the fluids. In this paper, we report on a scalable UV-ozone (UVO) treatment technique to produce highly stable dispersions for the elevated temperatures experienced by working fluids in radiant heating systems. To probe suitability of UVO treated multi-walled carbon-nanotube (MWCNTs) for solar-assisted radiant heating systems, this paper investigates the effects of exposure time and temperature on stability, optical absorbance properties, the extent of functionalisation, and the photothermal conversion performance of UVO-treated MWCNT nanofluids. No agglomeration or degradation of the MWCNTs was observed at elevated temperatures (up to 150   °C), highlighting the stability of proposed nanofluids. [ABSTRACT FROM AUTHOR]

The following paper presents a simple method of determining the presence, distribution and values of heat losses from external building walls as thermodynamic partitions using a Thermal Imaging Camera (TIC). According to Fourier's equation, the value of heat loss is proportional to the temperature gradient ∂ t /∂ y | y=0 in air in the y direction perpendicular to the heated surface. Unfortunately, air temperature cannot be measured with a TIC, as gases do not emit thermal radiation. It is therefore suggested that a grid placed vertically in the air and perpendicularly to the heated surface should be used as a detector of temperature field. Warmed by convective air flows, such a grid becomes a source of infrared radiation. The temperature distribution reproduced on the grid constructed from thin strands of a low thermal conductivity material is sufficiently distinct and sharp to be recorded with a TIC. The correctness of the TIC method has been tested in laboratory conditions on the example of a vertical heating plate, as well as in real-life conditions − on a single pane of glass serving as a building partition. When compared to the works of other authors, the result obtained as Nu  = 0.571· Ra 0.25 relation reveals a satisfactory correlation for the heating plate, while estimated temperature distributions and heat fluxes from the room to the surface of thermodynamic partition wall Q loss,conv,in and on the outside of the building Q loss,conv,ext successfully and quite precisely correlate (5–7% for both internal and external sides) with theoretical calculations. Therefore, the TIC method proposed in this paper can significantly extend the range of application of TICs in energy audits, especially for buildings [ABSTRACT FROM AUTHOR]

This paper and its future companion papers, were aimed to develop a set of efficient and accurate enough modular tools to build a simulation test bed for the stability analysis of interacting VAV (variable air volume air-conditioning system) control loops. By providing the correlation equations between the heat flow and temperature of the two opposite surfaces, the wall model was expected to link adjacent zones effectively and efficiently. In part 1, the development of an optimal simplified 3R2C (composed of three resistances and two capacitances) thermal network model of building envelope, i.e., the wall model, was descripted. The verification showed that the simplification of one-dimensional heat conduction through a typical wall of heavy construction was acceptable. The accuracy of the optimal simplified 3R2Cmodel was satisfying, and outperformed that of the optimal 4R3C model as well as two 3R2C models which were normally used in practical applications. First and foremost, the room dimensions of the test bed with the boundary conditions in numerical scheme were designed under the principle of similarity. After validation of the numerical scheme with the high-cited scaled experiment, a series of numerical test cases simulating the potential oscillations in practical applications, were conducted. Then, the long neglected concerned frequency range within which heat flow transfer through extern wall, was determined. Finally, the part of solid wall was separated with the part of indoor air, and was developed into a flexible wall module of 3R2C model based on frequency domain regression. [ABSTRACT FROM AUTHOR]

The seawater source heat pump (SWHP) is a renewable energy utilization system. The beach well infiltration intake system (BWIS) effectively improves the stability, reliability, and energy efficiency of SWHP systems in cold climate areas. BWIS research requires a multidisciplinary approach that involves both hydrogeology and heat transfer theory. There are many factors influencing the energy consumption and life-cycle costs of the systems, with each factor having many possible values. This paper aims to determine the degree of influence of ten representative factors of BWIS: specific heat capacity of rock-soil, rock-soil density, thermal conductivity of rock-soil, void fraction of rock-soil, rock-soil permeability, beach well radius, site length, site width, row number of beach wells, and column number of beach wells. In this paper, a BWIS seepage and heat transfer model is established. Based on the model, an orthogonal design optimization method is presented and analyses are conducted for heat pump unit energy consumption, seawater pump energy consumption, and economic cost for various conditions using MATLAB code. The results indicate that a greater number of beach wells is not always better and the number and placement of beach wells should be scientifically and objectively optimized by the orthogonal design method. The optimization method can scientifically guide engineering design for BWIS and offset the design deficiency that results from the current practice of only using a hydrogeological report of randomly selected test wells. [ABSTRACT FROM AUTHOR]

Energy pile is a novel ground heat exchanger (GHE) of ground source heat pump (GSHP) system, and the spiral coils are set into pile to form this type of GHE. However, the on-site heat transfer experiments combined with engineering projects are not much, and the corresponding validation of model is a little. This paper describes both heat transfer experiments and models of energy pile. During the experiments, some parameters of circulating fluid of spiral coils are recorded, and meanwhile the thermal resistors are installed at the pile’s surface to observe the temperature response induced by spiral coils. The heat transfer models of energy pile are explained and the corresponding characteristics are investigated. Afterwards, the comparisons between experimental data and model’s results are made to explore the differences, the difference is small and therefore the heat transfer models can be validated. Thus, the validation is a significant contribution and the model can provide guideline for project design; the research of the paper is helpful to promote further development of GSHP system. [ABSTRACT FROM AUTHOR]

Energy efficiency retrofit of buildings represents a key effort in reducing EU energy demand by 20% by 2020. However, predictions tend to overestimate savings by large percentages. The shortfall in savings can be attributed to incorrect predictive techniques, comfort takeback along with other behavioural and workmanship variables. Common predictive techniques related to heat loss tend to be based on the U-value of the building envelope. This paper presents the design of a more straightforward and compact version of the traditional Hot-Box apparatus (measures U-value) which instead determines the thermal resistance of samples of building envelope. U-value includes the need to measure/predict the effective surface resistances. In situ surface resistances, which include radiation and convection, are difficult to predict and vary depending on climatic conditions, exposure levels, surface emissivities among many other influences. The design of the test facility eliminates the need to incorporate these surface behaviour variables. This paper details the replicable apparatus and test methodology. The results of testing a hollow block wall of typical construction using the rig is then presented. The determined R-value is found to be within 1% of calculated values and the thermal time constant also matches closely with the most accurate predictive estimates. [ABSTRACT FROM AUTHOR]

Demand response (DR) of air-conditioning systems is important to shift or reduce the peak electricity demand of commercial buildings by shift or reduce the cooling load. Popular DR strategies of air-conditioning systems include zonal temperature reset and direct control of the main equipment. Many DR studies have been conducted on the thermal inertia of buildings for temperature resetting, but there are few studies on the thermal inertia of air-conditioning systems, which is relatively small but not negligible. In this paper, the thermal inertia of air-conditioning systems is defined as the character that causes the variation of the supply cooling capacity to zones lagging behind the variation of the cooling capacity from plants after DR strategies are implemented. This paper develops an inertia model of chilled-water systems with three sub-models, including chiller model, chilled-water pipe model and cooling coil model. The model describes the dynamic process from the cooling plant to terminal units when DR strategies on chillers are implemented. A new parameter Q ( t ) named the “refrigerant cooling capacity” is introduced in this study to simplify the thermal inertia model. The Q ( t ) patterns during the dynamic processes of two series of common chiller-side control strategies (On/Off control and resetting the chilled-water temperature) are obtained and validated using experiments and field tests. In the end, the entire transient model of air-conditioning systems is validated using experiments. [ABSTRACT FROM AUTHOR]

HVAC systems in buildings are composed of many subcomponents which consume energy in huge amounts. In recent years, energy consumption becomes very important aspect due to the lack of energy resources and environmental reasons. In terms of energy consumption, the components of HVAC systems account for large portion of total energy used in buildings. Therefore optimization of energy consumption in HVAC applications is an essential requirement in terms of thermodynamic and economic point of views. Besides, the efforts for decreasing energy consumption turn into economic recovery. It also provides benefits for human health and cleaner environment. In this paper, as originality, thermodynamic and economic analyses of an existing social and cultural building which has a heating and cooling area of 8852 m 2 are presented by comparing of a conventional HVAC and a VRF system. A novel contribution is given to the open literature by comparing two systems with actual data measured from the existing system. The other originality is to apply and test a VRF system to a complex-structured building. This study is a rare one which contains insulation, heating and cooling accounts of a building together. In this paper, VRF system is modelled to the existing building for the comparison of the conventional HVAC system. Both systems have been compared in terms of heating and cooling capacity, initial, operation and maintenance costs. It is found that the VRF system is more economic and efficient such that the VRF system is found to have 44% cost profit when compared with the conventional HVAC system. [ABSTRACT FROM AUTHOR]

This paper shows the investigation of the features of façade greening, concerning the reduction of heat demand in the winter. Supporters of façade greening claim that their systems can reduce the heat demand in the winter. But so far, there is no proof for that and each greening system is different. Two greened façades have been researched and measured for one heating season. The measurements were compared with parts of the same buildings which are not greened. So far there does not exist a general method, how to calculate the U-value at greened facades. The method which is developed in this paper makes the comparison between the greened part of the façade and the not greened part possible. It turned out that there is a difference of the thermal resistance between greened and not greened parts of the façades in winter. It is between 0,31 m 2 K/W and 0,68 m 2 K/W depending on the greening system and its location. [ABSTRACT FROM AUTHOR]

In recent years, with the rapid development of cities across the globe and global warming, Urban Heat Island (UHI) and building cooling energy consumption have reached a point of perilous concern. The use of shading systems on building fenestrations has long been known as an effective strategy to minimise façade heat gain as well as glare, resulting in lower cooling energy consumption and indoor comfort. Expending the potentials of shading systems beyond conventional use, the present paper focuses on the effects of shading systems on opaque facades that is currently unaccounted for in simplified Overall Thermal Transfer Value (OTTV) calculations in standards adopted by many countries. A field study was first conducted to compare the effects of various shading configurations on an opaque façade. Subsequently, a simulation study was then conducted to quantify the effects of shading systems on opaque facades with varying thermal transmittance value (U-Value). The results of this study revealed numerous benefits for the deployment of shading systems on opaque building facades, which includes, reduction to building conductive heat gain, building cooling loads, microclimate temperature near the façade, as well as potential reduction to carbon emissions over the building lifecycle. Moreover, the paper also provides considerations for the selection shading configurations, as well as the considerations that would be required for the formulation of a more accurate simplified OTTV equation that would take into account the effects of shading systems on opaque facades. [ABSTRACT FROM AUTHOR]

This paper describes an experimental set-up used to validate predictions from a numerical model for ventilated wall cavity with spray evaporative cooling systems. The validation analysis results have indicated that the numerical model, developed and described in a companion paper, can accurately predict the outlet air temperatures and relative humidity levels achieved by a ventilated wall cavity with a spray evaporative cooling system utilized to air condition buildings in hot and dry climates. Air velocity predictions of the ventilated cavity have been also verified against results obtained from a detailed CFD analysis. Moreover, the experimental testing results indicate that the ventilated wall cavity with an evaporative cooling with 5 μm spray droplets can significantly reduce outdoor air temperatures. Specifically, the system was able to reduce air temperature from 45 °C to 25.5 °C. Moreover, the results of a parametric analysis obtained using the validated numerical solution suggest that there is a significant reduction potential in heat transfer through cavity walls while providing acceptable supply air temperature capable of maintaining indoor thermal comfort. In addition, the parametric analysis has evaluated the effect of climatic conditions (temperature and humidity) on the required distances for spray droplets to fully evaporate. The presented results could help in designing effective and energy efficient ventilated wall cavity with spray evaporative cooling systems to condition buildings in hot and dry climates. [ABSTRACT FROM AUTHOR]

Due to low cost, light weight and corrosion resistant features, polymer heat exchangers have been extensively studied by researchers with the aim to replace metallic heat exchangers in a wide range of applications. Although the thermal conductivity of polymer material is generally lower than the metallic counterparts, the large specific surface area provided by the polymer hollow fibre heat exchanger (PHFHE) offers the same or even better heat transfer performance with smaller volume and lighter weight compared with the metallic shell-and-tube heat exchangers. This paper presents the construction and experimental investigations of polypropylene based polymer hollow fibre heat exchangers in the form of shell-and-tube. The measured overall heat transfer coefficients of such PHFHEs are in the range of 258–1675 W/m 2 K for water to water application. The effects of various parameters on the overall heat transfer coefficient including flow rates and numbers of fibres, the effectiveness of heat exchanger, the number of heat transfer unit (NTU), and the height of transfer unit (HTU) are also discussed in this paper. The results indicate that the PHFHEs could offer a conductance per unit volume of 4 × 10 6 W/m 3 K, which is 2–8 times higher than the conventional metal heat exchangers. This superior thermal performance together with its low cost, corrosive resistant and light weight features make PHFHEs potentially very good substitutes for metallic heat recovery system for building application. [ABSTRACT FROM AUTHOR]

Fenestration systems are important elements of building facades. Although there are fenestrations placed at different angles in facades, not only vertical and roof glazing, there is little information about calculating procedures of thermal resistance and thermal transmittance for reversed glazing in which heat flows in a downward direction. The aim of this paper is to propose some amendments to EN 673–Glass in building – Determination of thermal transmittance (U value) – Calculation method [1] as well as to ISO 15099 – Thermal performance of Windows – Detailed calculations [2] to improve calculation accuracy for glazing under downward heat flow direction. In the paper CFD modeling of glazing thermal transmittance is presented. CFD calculated data were then validated by measurements performed in a calorimetric chamber in a test stand prepared for thermal transmittance measurements under different glazing angles resulting in a downward heat flow direction. Measurement data are then compared to simulation results. After achieving satisfactory agreement, some additional simulation results are presented and some amendments to glazing thermal resistance calculation procedure given in EN 673 and ISO 15099 standards are proposed. [ABSTRACT FROM AUTHOR]

This paper builds three-dimensional heat transfer model of single- and double-U type vertical ground heat exchanger (GHE). The heat transfer and pressure loss of vertical GHE is simulated with different flow velocities, pipe diameters and well depths. Moreover, the value engineering (VE) is used to evaluate the tech-economic performance of the buried pipe. The value coefficient of each scheme is compared with selecting five functional indicators. The objective of this paper is to obtain the recommended structures for the GHE in engineering applications. The results show that single-U32, double-U25 and double-U32 have their own advantages in the aspects of the heat transfer, outlet temperature, pressure loss and material consumption. The recommended velocity ranges of single-U32, double-U25 and double-U32 are 0.4–0.6, 0.4–0.5, 0.3–0.4 m/s respectively. Finally, the recommended well depth ranges of double-U25 and double-U32 are 80–100 m and 90–110 m respectively. [ABSTRACT FROM AUTHOR]

A model predictive control (MPC) has been implemented in a medium-sized office building in Brussels, Belgium. This paper presents the implementation of the controller and the measured performance in comparison with the default, rule-based control (RBC). The building has two floors and a total size of 960 m 2 . The controllable system is the hybrid heat production consisting of two air/water heat pumps and a condensing gas boiler. The practical situation does not allow controlling end-units in the different zones of the building. The MPC makes use of a Modelica grey-box control model resulting from a system identification with monitoring data. The paper covers the monitoring, model identification, forecasting of disturbances, state estimation, formulation and solving of the optimal control problem (OCP) and transmission of the control signals. The performance is evaluated on a daily basis based on analysis of heating degree days, thermal comfort, energy costs and primary energy consumption. The results show that the model predictive controller is able to provide a similar or better thermal comfort than the reference control while reducing the energy costs by more than 30%. This is due among others, to a better use of the heat pumps and an adapted hot water supply temperature. [ABSTRACT FROM AUTHOR]

Improving the performance of air-to-air heat recovery systems, as measured by supply air temperature efficiency, is an important energy saving strategy that is often regulated by building codes. The high nonlinearity of supply air temperature efficiency with airflow rate in a run-around heat recovery system makes the trend prediction of supply air temperature efficiency especially challenging for field measurement. This paper proposes a simple and novel field measurement based methodology, supported by the power law relationship of air-side heat transfer, to evaluate the performance of run-around heat recovery systems. A system dependent power, signature power , is proposed that establishes a linear relationship between the supply air temperature increment across the supply air heat exchanger and a parameter—the maximum temperature difference between exhaust and supply airstreams divided by the signature power of the supply airflow rate. This methodology can predict the supply air temperature efficiency and is verified using four run-around heat recovery systems with field measurements. This new methodology can possibly be applied to other types of air-to-air heat recovery systems. This paper also describes a tuning method for determining the signature power based on field measurements and addresses the heat recovery efficiency of run-around heat recovery systems. [ABSTRACT FROM AUTHOR]

Glazing energy resulting from solar radiation can be a primary source to vary the thermal field inside of a building. As the glass material is loaded by excessive solar radiation, the drastic increase in the glazing energy yields the greater solar heat transferring indoors and thereby raises the interior temperature. Reducing the glazing energy or temperature resulting from solar radiation can be a solution to decrease the solar effect on the interior thermal field and subsequently advantageous to reduce energy demands. Therefore, this paper provides non-linearly patterned glass technology which incorporates non-linear patterns throughout the exterior surface of glass to reduce the solar effect on the glass material. Based on theoretical and experimental analyses provided in this paper, incorporating non-linear patterns over the glass surface is able to yield the increase in the incident angle as well as the decrease in the solar energy acting on the glass. Therefore, the temperature reduction of the solar-loaded glass material can be acquired as the non-linear pattern is applied. However, the thermal performance of non-linearly patterned glass is dependent on the pattern design. Reducing the dimension of the pattern spacing and/or the radius of curvature of the non-linearly pattern member helps decrease the surface temperature of glass under solar radiation. [ABSTRACT FROM AUTHOR]

This paper numerically investigates the performance and heat and mass transfer processes that occur in counter-flow indirect evaporative air coolers. Different types of counter-flow air coolers were compared: a typical counter-flow unit, the same unit operating as a heat recovery exchanger, a regenerative unit and a novel modified exchanger, proposed by authors. Additionally the paper investigated impact of location of the perforated holes in the channel plate of regenerative air cooler. The numerical model is based on a one-dimensional heat and mass transfer analysis that was validated against existing experimental data. The results obtained from the simulation reveal the high effectiveness of the presented units. It was established that selected arrangements of the presented exchangers are characterized by the different efficiency in different air-conditioning applications. [ABSTRACT FROM AUTHOR]

Displacement ventilation (DV) flows are more complex than conventional overhead mixing systems since the stratified room environment cannot be modeled using the traditional fully mixed room air approach. A successful DV designer must be able to control the vertical room temperature profile and manage the position of the lower boundary of the upper air layer that contains heat and pollutants. The inclusion of a chilled ceiling (CC) in the DV system increases the complexity by adding the need to manage the CC cooling power so that it does not disrupt the DV stratification. This paper presents the extension of an existing DV nodal model so that the effects of the CC in room airflow and air temperatures can be predicted. The model uses three air nodes and focuses on the thermal plumes as the drivers of the airflow and room air heat exchange. The proposed model is validated using twelve different test chamber configurations from three independent experimental studies. When compared with existing models the proposed model achieves improved precision and model flexibility while using less air nodes. The last section of the paper presents a set of CC/DV design charts that can assist system designers in early design phases. [ABSTRACT FROM AUTHOR]

Autoregressive models with exogenous (ARX) are widely used for parameters identification of walls although their parameters do not have a direct physical interpretation. This paper uses measured data and an ARX model, obtained by deduction from a thermal network, for identifying the parameters with physical meaning of a wall tested under real weather conditions. The physical parameters are the U -value, the dynamic solar energy transmittance and the effective heat capacity. The paper presents the whole chain of transformations from thermal networks to ARX models for demonstrating that the procedure used for recovering the physical parameters guarantees their unicity. The estimations of the physical parameters are presented together with their uncertainties. [ABSTRACT FROM AUTHOR]

Stochastic building occupancy models are increasingly used to underpin building energy demand models, especially those providing high-resolution electricity demand profiles. This paper describes the development of an established two-state active-occupancy model into a four-state model in which the absent/present state and the active/inactive state are treated separately. This provides a distinction between sleeping and absence and so offers an improved basis for demand modelling, particularly high-resolution thermal modelling. The model uses a first-order Markov chain technique and the paper illustrates the value of this approach in duly representing the naturally occurring correlation of occupancy states in multiply occupied dwellings. The paper also describes how the model has been enhanced to avoid under-representation of dwellings with 24 h occupancy. The model has been implemented in Excel VBA and made available to download for free. The model is constructed from and verified against UK time-use survey data but could readily be adapted to use similar data from elsewhere. [ABSTRACT FROM AUTHOR]

To study the dynamic thermal response performance of concrete radiant cooling slab, this paper introduces the concept of core temperature layer. Heat transfer process in the concrete slab is divided into three sub-processes, correspondingly, and three heat transfer models are built by reaction coefficient method. Two-dimensional unsteady heat transfer model of concrete cooling slab is established ultimately. Experiments are conducted to test the slab thermal performance in steady and unsteady conditions. Error analysis and consistency verification are presented between the experimental value and calculated results of the model. The relative error between calculation and experimental value is within 2% in steady conditions and within 7% in unsteady conditions. The application scope of the heat transfer model for practical engineering is defined in this paper. [ABSTRACT FROM AUTHOR]

• Use of Active Thermal Insulation allows to reduce heat loss through the opaque walls by 60%. • Active Thermal Insulation captures, stores and uses the sun's energy (island system). • The glycol temperature in the ATI system was systematically lower than the internal temperature and higher than the external temperature. • The ATI system in the building shortens the heating season. • Using the ATI system allows to reduce the thickness of the external partitions. Active thermal insulation (ATI) is a sustainable technical solution in which the thermal energy stored in the ground is used to reduce heat transfer through the external building envelope. The authors proposed an innovative combination of the ATI system with the EVG-3D structural system, which consists of the panels with a layer of galvanized mesh on both sides of the polystyrene core, connected by the galvanized steel trusses that pass through the polystyrene core. The paper analyses the energy efficiency of the EVG-3 wall with the ATI system. The results were monitored for ten years (2012–2022) in a demonstration house in north eastern Hungary. One heating season was selected for more detailed analysis. The mean measured reduction of the heat flux lost through the building's opaque external envelope in this season was 60.3% with the maximum monthly reduction equal to 73.1% at the beginning of the heating season. The unique nature of the presented here results lies in the fact that they are based on very long and continuous measurements of the innovative solution, carried out in a real building. [ABSTRACT FROM AUTHOR]

Abstract: The present paper focuses on the development and experimental validation of a model of air-to-air heat exchanger dedicated to domestic mechanical heat recovery ventilation. The proposed model describes dry and partially wet regimes. The first part of the paper presents a semi-empirical model based on the physical characteristics of the heat recovery device and relying on empirical correlations available in the literature for the convective heat transfer coefficients. In the case of partially wet regime, a moving boundary model is applied in order to predict sensible and latent heat transfer rates. A model developed with friction factor coefficients estimated by correlations from the literature is also presented in order to predict the hydraulic performance in dry conditions. The second part of the paper describes the experimental investigation conducted on an off-the-shelf heat exchanger. Experimental data are used to tune correlations for the determination of the convective heat transfer coefficient and validate the proposed simulation model of the ventilation heat recovery exchanger in partially wet conditions. The model developed to determine the hydraulic performance with existing correlations for the friction factor coefficient does not require a calibration. Finally, examples of use of the developed model are presented, which includes coupling the model with a building simulation model, a study of the influence of the humidity on the evolution of the latent and sensible heat transfer rates and strategies to avoid freezing in the heat exchanger. [Copyright &y& Elsevier]

Abstract: The paper presents the full-scale experimental investigation of the dynamic heat storage potential of the prefabricated hollow core deck elements with and without phase change material (PCM) and with and without increased bottom surface area of the decks. In the presented investigation five types of hollow core decks with different surfaces on the bottom are investigated: reference deck made of standard concrete and flat surface, deck with special mortar grooved tiles, deck with flat mortar tiles, deck with grooved mortar and phase change material tiles, deck with flat mortar and phase change material tiles. The experimental investigation presented in the paper is performed in the specially designed modified hot box apparatus that allows maintaining periodic steady-state tests with the full-scale concrete deck elements. The presented research investigates if the extended surface area and PCM can result in the increased heat amount that can be transferred and stored in the heavy construction element during the diurnal indoor temperature fluctuations. [Copyright &y& Elsevier]

Abstract: Nowadays designers have to create energy efficient systems. Besides, they have to provide thermally comfortable environments for occupants. These, sometimes conflicting requirements lead to the development of such systems like surface heating and cooling systems that affect occupant''s heat exchange with his surroundings via radiation. With regards to the above described systems, combined effect of two local discomfort parameters is studied in this paper, namely radiant temperature asymmetry and warm floors. The European standard CR 1752 deals with these parameters separately and no data are available on how many people are dissatisfied due to combined exposures. The paper summarizes the results of climate chamber experiments conducted with a thermal manikin as well as human subjects that were exposed to cold wall and warm floor simultaneously present. The results show that equivalent homogenous temperature (EHT) of the manikin decreased by 2°C at the hands and face due to the cold radiating surface regardless of the warm floor. It was found that the thermal sensation and skin temperatures of human subjects were affected more by the radiation from the vertical cold surface than by the warm floor. Furthermore, genders showed significant differences regarding skin temperature change during sessions. [Copyright &y& Elsevier]

Abstract: This paper computes the thermal delay in multilayer systems. The heat transfer by conduction across multilayer systems under unsteady boundary conditions is simulated using analytical solutions. The solutions, obtained as Green''s functions, are established by imposing temperature and heat flux continuity at the media interfaces of the various layers. The unsteady state conditions are dealt with by first computing the solution in the frequency domain and then applying (fast) inverse Fourier transforms into space-time. After validating the analytical formulation by comparing these results with those obtained experimentally, the thermal delay is computed for different multilayer systems. It is computed as the difference between the time the thermal variation is recorded in one of the system''s surfaces and the time it appears at the opposite surface. The paper calculates the computed thermal delay for construction walls made of cork, mineral wood and extruded polystyrene panels. The thermal properties of these materials have been previously defined experimentally. In the simulations, the systems are subjected to temperature changes that vary according a sine function, with the aim of defining the environmental exterior temperature variation over a period of several days. [Copyright &y& Elsevier]

Abstract: This paper reports on the development, experimental validation and application of a semi-empirical model for the simulation of the phase change process in phase change materials (PCM). PCMs are now increasingly being used in various building materials such as plasterboard, concrete or panels to improve thermal control in buildings and accurate modelling of their behaviour is important to effectively capture the effects of storage on indoor thermal conditions. Unlike many commercial simulation packages that assume very similar melting and freezing behaviour for the PCM and no hysteresis, the methodology employed treats the melting and freezing processes separately and this allows the inclusion of the effect of hysteresis in the modelling. As demonstrated by the results in this paper, this approach provides a more accurate prediction of the temperature and heat flow in the material, which is of particular importance in providing accurate representation of indoor thermal conditions during thermal cycling. The difference in the prediction accuracy of the two methods is a function of the properties of the PCM. The smaller the hysteresis of the PCM, the lower will be the prediction error of the conventional approach, and solution time will become the determining factor in selecting the simulation approach in practical applications. [Copyright &y& Elsevier]

Abstract: The article described the dilatancy technology of district heating system with high-temperature heat pump (HTHP). For the exploitation of low-temperature return network sources, a high-temperature heat pump with a heat exchanger was planned. As the core technology of the dilatancy district heating system, HTHP units and heat exchanging systems had been described detailedly through theoretical and experimental analysis. The model was proved to be in good agreement with the experiment data. It could be used for engineering design. In this paper the district heating system in Datong was taken as a case study. The main characteristics of the dilatancy district heating system had been presented in the form of diagrams. The principal advantage of dilatancy technology in the district heating system lies in increasing the temperature difference, reducing the diameters and the initial investment of primary side network. This paper shows clearly the analysis of the dilatancy technology for characterising and improving the performance of district heating systems. [Copyright &y& Elsevier]

Abstract: Thermal mass can be effectively used for controlling temperature in buildings. In this paper, the dynamic heat transfer performance of interior Planar Thermal Mass (iPTM) subject to sinusoidal heating and cooling is investigated. Analytical solutions of temperature distribution and heat flux in iPTM are deduced. It is shown that the effective heat storage capacity of iPTM reaches maxima at an optimal thermal mass thickness. Therefore, from the consideration of building material as thermal mass rather than as structural material, there is no rationale of using thickness greater than optimal thickness.The convective effect at iPTM surface is analyzed. A virtual average heat flux and an effective thermal resistance are defined. It is concluded that, for most common indoor building materials, the convective effect cannot be neglected. Because of the convective effect and the nonlinearity of the heat storage of iPTM, it is shown that the lumped method, which is used to do interior thermal mass calculation in most papers and softwares, is incorrect under most circumstances. [Copyright &y& Elsevier]

Abstract: Vertical U-tube ground heat exchangers (GHEs) are a key component in geothermal energy utilization systems like ground source heat pumps (GSHPs). This paper presents a new constant heat flux model of its. Different from the existing uniform constant heat flux (UCHF) model, the model proposed in this paper is a cumulative constant heat flux (CCHF) model. The model is developed by reconstructing an experiment-verified three-dimensional unstructured finite-volume model. Applying the model, the borehole resistance with any geometric structure can be found. In addition, it may be used to develop the short time-step temperature response factors (TRFs) for simulating the behavior of the GHE on a short-time-step basis (one hour or less). [Copyright &y& Elsevier]

Abstract: As solar gains often constitute a high fraction of the overall cooling and heating loads in buildings, dynamically and adaptively controlled fenestration may be an important strategy for energy efficient buildings. The higher the variability in climatic conditions over the seasons, the longer is the list of benefits deriving from the adoption of this type of technology. This paper reports the experimental evaluation performed on a dynamic shading system based on a liquid layer sliding within a dedicated glass stratification. The advantages connected with the adoption of the described technology derive from its easy adaptability to almost every fenestration, its reversible switching between high and low solar transmittance configurations in a short time and its capability to maintain transparency in both operating modes. The summer outdoor experimental campaign, reported in this paper, allowed the solar factor of the liquid-shaded stratification to be estimated and compared with a reference low-emission double stratification, where one of the panes was treated with a magnetronic coating. The results showed not only that the shading provided by the dynamic liquid layer is more effective than its benchmark, but also that its dynamic capabilities have been preserved after a summer-long exposure to solar radiation. [Copyright &y& Elsevier]

Highlights • A geopolymer cube from fly ash, ground granulated blast furnace slag and dune sand is produced. • Expanded clay containing phase change materials is integrated to the cube. • The cube is tested for structural and thermal performance. • The PCM containing cube showed improvements in thermal performance compared to reference. • The PCM containing cube lost structural strength compared to reference. Abstract In this paper, novel geopolymer-coated expanded clay-phase change material (GP- L -PCM) macrocapsules added to geopolymer concrete (GPC) with volume ratios of 25%, 50%, and 75% for the production of GP- L -PCM slabs are described. The same amounts of lightweight expanded clay aggregate (LECA) were used to develop LECA slabs. The thermal and structural performances of LECA and GP- L -PCM slabs were compared with a reference GPC slab. It was observed that LECA and GP- L -PCM slabs exhibit lower heat transmission, represented by lower maximum surface temperatures (a decrease of 5.6°C and 8.0°C on the back surface) than that of GPC slab. Accordingly, the LECA and GP- L -PCM slabs exhibit U-values of 1.6 and 0.9 W/m2K, respectively, with 75% addition of LECA and GP- L -PCM capsules compared to 2 W/m2K for GPC. Substantial decrease in compressive strength was observed in the LECA and GP- L -PCM slabs as compared to GPC for all tested samples. [ABSTRACT FROM AUTHOR]

Highlights • The challenge of installing aerogel-enhanced blankets are investigated. • Mechanical fasteners have considerable effect over the effective thermal properties of aerogel-enhanced blankets. • Equivalent thermal conductivities are experimentally measured and numerical calculated. • A moderate use of steel fasteners increases the effective thermal conductivity of the aerogel-enhanced blanket by over 50%. • Calculated values for any concentration of plastic and metal fasteners are reported. Abstract The use of silica aerogel-enhanced blankets as a thermal insulation material is one of the most promising solutions to reduce the heat loss through the building envelope. Aerogel-enhanced blankets provide exceptionally high thermal resistance, thanks to a thermal conductivity value as low as 0.014 W/mK. However, a challenge in the use of aerogel-enhanced blankets is represented by the way to connect these highly flexible panels to the backing support. This paper describes the effect of mechanical fasteners over the thermal performance of a brick wall covered with an aerogel-enhanced blanket investigated. The effective thermal transmittance of the selected aerogel-enhanced blanket due to the presence of the metal fasteners is evaluated experimentally using a calibrated chamber. The laboratory measurements are also compared with theoretical calculations conducted both for metal and plastic fasteners. Finally, some conclusions about the ways to reduce the thermal bridging effects during the installation of aerogel-enhanced blankets are provided, and the effective thermal conductivity of aerogel blankets considering different materials (plastic or metal anchors) for their fastening are reported. [ABSTRACT FROM AUTHOR]

Highlights • The practical energy performance of MD-GHPs applied for space heating is introduced. • The MD-GHPs reduce the space occupation and eliminate thermal imbalance of the ground. • The average temperature of the outlet water from the ground can reach 33.0°C. • The COP of the heat pump reaches 5.43 and the COP hs of heat source reaches 4.58. Abstract This paper introduces the practical energy performance of medium-depth geothermal heat pump systems (MD-GHPs), which use vertical concentric tube as ground heat exchangers (GHEs) to extract heat from the medium-depth geothermal energy for space heating in buildings. Hereby the medium-depth geothermal energy refers to heat embodied in rocks nearly 2–3 km underground with temperature around 70–90°C. The operation performance and energy efficiency of MD-GHPs is tested in five residential buildings in Xi'an, North China. According to the short-term and annual field test results, the energy performance of MD-GHPs as well as the heat transfer performance of GHEs are introduced and analyzed. Results show that the average temperature of the outlet water from the ground can reach 33.0 °C, promoting the performance of whole systems greatly, with the COP (Coefficient of Performance) of the heat pump reaching 5.43 and the COP hs of heat source reaching 4.58. Therefore, the energy saving rate (ESR) of the MD-GHPs can reach 41.0%, 29.3% and 47.5% compared with air source heat pump systems (ASHPs), conventional ground source heat pump systems (SD-GHPs) and gas boilers. [ABSTRACT FROM AUTHOR]

Highlights • Thermal quadrupoles formalism is applied for modelling transient heat transfers in building walls with a semi-infinite assumption. • Front face response curves of multi-layered walls are analysed to estimate the thermal properties of the wall. • Experiments are carried out on two classical multi-layered building walls using artificial thermal excitations. • The proposed method reduces the evaluation time less than 10 hours. Abstract Thermal performances of building walls are significant for energy conservation. However, very few non-destructive evaluation methods exist to quantitatively diagnose the building walls in situ due to the walls' large thickness. Moreover, most of the existing methods are inconvenient to implement in situ and take a long characterization time. This paper studies transient heat transfer to estimate the wall's thermal properties based on the thermal quadrupoles modelling. Semi-infinite boundary condition is assumed at the rear face of the wall. With this assumption, only the front face response of the wall is considered. The evaluation time is then effectively reduced within a few hours, and the diagnosis in situ is simplified without the measurement on the rear face of the wall. Experiments are carried out on two traditional multi-layered building wall cases using heating lamps. With the measured surface temperatures and heat fluxes, the unit-pulse response and unit-step response at the front surface of the investigated wall are reconstructed through a deconvolution approach and a TSVD (Truncated Singular Value Decomposition) inversion. The unit-step response curve is directly characterized by the thermal resistance, thermal effusivity and heat capacity of the wall, thus allowing us to estimate the wall properties. The characterization time for the two cases is less than 10 hours. [ABSTRACT FROM AUTHOR]

Highlights • Analysis of the average temperature dependency of thermal conductivity in VIPs. • Analysis of the heat transfer contributions that affect thermal conductivity in VIPs. • Evaluation of the combined effect of ageing and temperature on thermal conductivity. • Impact of thermal conductivity variation on thermal behaviour of a VIP-based roof. • Measures to mitigate severe conditions for VIP-based components were introduced. Abstract Over the last few years, the adoption of Vacuum Insulation Panels (VIPs) in building envelopes has increased. However, in order to obtain a correct implementation of VIPs in buildings, it is crucial to conduct a proper analysis of the thermal bridging, the service life and the ageing effects at both the design stage and during building operation. A further factor that should be considered is the dependency of thermal conductivity on temperature. In this paper, an experimental campaign has been carried out to evaluate the variation in the thermal conductivity of VIPs with the average temperature and to qualitatively assess the heat transfer contributions that affect this variation. The study has also been devoted to evaluating the effect of a variation in the thermal conductivity considering various VIP ageing stages. Moreover, dynamic heat transfer simulations have been performed, using a validated model, to investigate the impact of considering a temperature dependent thermal conductivity on the overall thermal behaviour of a building roof with VIP-based insulation. [ABSTRACT FROM AUTHOR]

Abstract: Currently, several whole-building simulation tools (e.g., esp-r, EnergyPlus, TRNSYS, TAS, IES VE, IDA ICE, VA114, BSim, etc.) are used to assess the energy performance of double-skin façade (DSF) buildings. The aforementioned tools are well suited to assess energy performance of conventional building systems or whole buildings; however, it is questionable whether such tools can accurately describe the transient heat and mass transfer phenomena that occur in the complex three-dimensional geometry of DSFs. This paper describes an empirical validation of the EnergyPlus simulation tool for performance simulation of a DSF. A series of experiments were conducted for cavity airflow and thermal behavior of the DSF and then compared with simulation outputs. In this paper, it is shown that there are significant differences in both thermal and airflow behavior of DSFs between the measurements and simulation predictions by EnergyPlus. This study investigates three cases causing the differences and elucidates what should be considered when modeling DSFs using EnergyPlus. [Copyright &y& Elsevier]

Abstract: This paper presents a review of studies on natural convection heat transfer in the triangular enclosure namely, in attic-shaped space. Much research activity has been devoted to this topic over the last three decades with a view to providing thermal comfort to the occupants in attic-shaped buildings and to minimising the energy costs associated with heating and air-conditioning. Two basic thermal boundary conditions of attic are considered to represent hot and cold climates or day and night time. This paper also reports on a significant number of studies which have been performed recently on other topics related to the attic space, for example, attics subject to localized heating and attics filled with porous media. [Copyright &y& Elsevier]

Abstract: Low and moderate geothermal resource can be found in most areas of the world and heat pump system as an efficient device to utilize the geothermal is prevalent. It is important to know temperature field around the heat-exchange pipelines for efficient conversion and use of geothermal energy. The temperature distribution around the heat pipe underground was studied by the heat source method in the paper. Based on the method, every heat pipe could be supposed as a heat source that would produce a corresponding temperature field, and the total temperature underground could be the superposition of every temperature- field. The temperature fields around heat pipes were given in the paper in different containing water rate of the soil, gap rate of the soil and different disposal of heat pipes. It would be beneficial for design and better operation of the heat pipe system underground to use geothermal resource. [Copyright &y& Elsevier]

Abstract: Heat is transferred through building envelopes by conduction, radiation, and convection. Of these, convective heat transfer is often the weakest ‘link’ in the overall heat transfer model. While conduction and radiation heat transfer are supported by well-established analytical and numerical models, the treatment of convection is much less rigorous. Convection modeling involves solution of fluid dynamics problems in which the complexity of an enclosed space''s geometry and the diversity of indoor airflow patterns require many fit-for-purpose convection correlations. These must take into account specifics of the space and of the heating ventilation and air-conditioning (HVAC) system. This paper provides an overview of the current knowledge on the modeling of convective heat transfer in load calculation and building energy simulation programs. The paper also discusses various issues related to the robustness of convection coefficient correlations, and reports on new experiments conducted to test the sensitivity of existing convection correlations. The discussed issues relate to: (1) the non-uniformity of the indoor temperature field; (2) the selection of a proper characteristic dimension; (3) the effect of adiabatic and non-adiabatic obstructions in the zone; and (4) the impact of airflow disturbance. [Copyright &y& Elsevier]

Abstract: An experimental investigation has begun concerning the hygrothermal behaviour of wooden frame houses. The experimental set-up, consisting in a full-scale wooden frame house exposed to natural exterior climate is presented in the paper. It is located in Grenoble (South-East of France), and is equipped with over 150 sensors in order to collect a large amount of data (temperatures, relative humidity). The experimental data presented in the paper enable to get better understanding of coupled hygrothermal phenomena, and can be used to allow the validation of numerical models for heat, air and moisture transfers in wooden frame buildings. The analysed results demonstrate the importance of temperature-driven moisture diffusion from hygroscopic panelling. [Copyright &y& Elsevier]

Abstract: This paper describes a new general method for building simulation programs which is intended to be used for the modelling of complex facades. The term ‘complex facades’ is used to designate facades with venetian blinds, prismatic layers, light re-directing surfaces, etc. In all these cases, the facade properties have a complex angular dependence. In addition to this, such facades very often have non-airtight layers and/or imperfect components (e.g. non-ideal sharp edges, non-flat surfaces, …). Therefore building planners often had to neglect some of the innovative features and to use ‘work-arounds’ in order to approximate the properties of complex facades in building simulation programs. A well-defined methodology for these cases was missing. This paper presents such a general methodology. The main advantage of the new method is that it only uses measureable quantities of the transparent or translucent part of the facade as a whole. This is the main difference in comparison with state of the art modelling based on the characteristics of the individual subcomponents, which is often impossible due to non-existing heat- and/or light-transfer models within the complex facade. It is shown that the new method can significantly increase the accuracy of heating/cooling loads and room temperatures. [ABSTRACT FROM AUTHOR]

Highlights • A new lab test is proposed to study dynamic energy systems and composites with PCM. • Transient plane source technique and controlled dynamic environment sensing are coupled. • Concrete with PCM for thermal energy storage in buildings are dynamically tested. • The test method allows to identify PCM dispersion and activation and TES contribution. Abstract In recent years the combination of latent and sensible thermal energy storage processes within passive and active building components has gathered increasing attention among researchers all over the world. All this considered, an ever-increasing hunger for bridging the gap between chemistry-based characterization methods and engineering-scale monitoring solutions is spreading worldwide, with the aim of developing new experimental procedures allowing to test these advanced systems under realistic operative environmental conditions. In this view, the present paper aims at proposing an innovative characterization procedure for real-scale engineered applications with reference to dynamically variable composites, such as PCM-doped cement-based mixes for building envelope applications. Therefore, in this work, thermally enhanced concretes were produced using micro-encapsulated paraffin and their activation was investigated by means of an innovative experimental technique coupling transient plane source method and controlled environmental dynamic forcing. The dynamic-TPS analysis allowed to produce temperature dependent profiles for three basic material thermal properties, i.e. thermal conductivity, thermal diffusivity and volumetric specific heat. Results show that the proposed methodology is capable of detecting PCM activation within the engineered composites during the imposed hygrothermal cycle, where, for example, the effective thermal conductivity of the composites varied between 40 and 90% amid the melting/freezing process. Furthermore, the transient plane source method also showed to be suitable to investigate PCM dispersion within the composites. [ABSTRACT FROM AUTHOR]

Abstract The globe thermometer has been considered a reliable instrument to quantify mean radiant temperature (MRT) since Bedford & Warner isolated its readings from air movement in their 1934 paper so that radiation could be quantified. Recent expanded use of radiant heating and cooling systems has presented new challenges for the usage of globe thermometers in the built environment by causing additional radiant asymmetries and performance expectations. Therefore, we replicate the original Bedford & Warner work to reconsider and develop a more holistic understanding of black globe performance and the determination of MRT in buildings. We recreate the MRT and air temperature separation to investigate the accuracy of globe thermometers on measuring MRTs. A radiantly heated open-plan laboratory and a radiantly cooled conference room were selected and measured with multiple globe thermometers and non-contacting infrared sensors. The globe temperature results were then corrected with air movement to produce MRTs and compared against MRTs simulated from measured surface temperatures. We demonstrate a significant impact of air speed on the MRTs obtained from globe thermometers. We also illustrate a less-investigated non-graybody emissivity variation and spatial variation of MRTs of up to 5 °C at the same height. We believe the increasing temporal and spatial resolution of digital sensors may create new challenges for using globe thermometers to measure MRTs, since fluctuating readings may camouflage potential MRT changes. Through a validation of our spatial MRT distribution with experimental results, we believe there is a need for better sensors that could spatially resolve MRTs, and recognize issues with both air speed and emissivity. [ABSTRACT FROM AUTHOR]

Highlights • The U -value impact on the thermal performance of residential buildings was analyzed. • 96,000 buildings were randomly generated and evaluated using dynamic simulation. • Six geometry-based indexes were correlated with the buildings performance. • Low U -values can increase energy consumption in warm and moderate climates. • Adequate U -values allow designers to explore new building forms and window designs. Abstract European countries impose regulations for low thermal transmittance envelopes to improve the buildings' energy efficiency. However, in scientific literature, evidences are surfacing that such low U -values are affecting the validity of traditional design guidelines. The purpose of this paper is to analyze the implications of lowering the envelope U -values. To achieve this, 96,000 residential buildings were generated, with random geometries and U -values, and their energy consumption evaluated for eight European locations. The buildings were grouped according to the envelope elements' thermal transmittance and the results statistically analyzed. For each group, six geometry-based indexes were correlated with the energy performance. As U -values decrease, the performance variation amplitude was found to reduce, making the geometry less important. However, in warm/moderate climates, low U -values tend to actually increase the energy consumption and also rise the performance variation, meaning that geometry regains importance. In this case, instead of helping reducing the heating demands, solar exposed windows and compact geometries raise the energy consumption. It is concluded that, for each climate location, there is an ideal U -value range for which the energy demand is low and the geometry effect becomes less significant, thus freeing designers to further explore building forms and window designs. [ABSTRACT FROM AUTHOR]